The present invention is generally related to components which comprise a high-resolution inkjet printer and is more particularly related to a printhead capable of a large number of dots-per-inch (dpi) placement of ink on a medium for a high-resolution printer.
Simply stated, inkjet printers operate by expelling a small volume of ink through a plurality of small orifices in an orifice plate held in proximity to a paper or other medium upon which printing or marks are to be placed. These orifices are arranged in a fashion in the orifice plate such that the expulsion of droplets of ink from a selected number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the orifice plate or the medium followed by another expulsion of ink droplets results in the creation of more segments of the desired character or image. Furthermore, inks of various colors may be coupled to individual arrangements of orifices so that selected firing of the orifices can produce a multi-colored image by the inkjet printer.
Several mechanisms have been employed to create the force necessary to expel an ink droplet from a printhead, among which are thermal, piezoelectric and electrostatic mechanisms. While the following explanation is made with reference to the thermal inkjet expulsion mechanism, the present invention may have application for the other ink expulsion mechanisms as well.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature that exceeds the boiling point of the ink solvent to create a vapor phase bubble of ink. Such rapid heating of the ink is generally achieved by passing a pulse of electric current, typically for one to three microseconds, through an ink ejector that is typically an individually addressable heater resistor. The heat generated thereby is coupled to a small volume of ink held in an enclosed area associated with the heater resistor and which is generally referred to as a firing chamber. For a printhead, there are a plurality of heater resistors and associated firing chambersxe2x80x94perhaps numbering in the hundredsxe2x80x94each of which can be uniquely addressed and caused to eject ink upon command by the printer. The heater resistors are deposited in a semiconductor substrate and are electrically connected to external circuitry by way of metalization deposited on the semiconductor substrate. Further, the heater resistors and metalization may be protected from chemical attack and mechanical abrasion by one or more layers of hard and non-reactive passivation. Additional description of basic printhead structure may be found in xe2x80x9cThe Second-Generation Thermal Inkjet Structurexe2x80x9d by Ronald Askeland, et al. in the Hewlett-Packard Journal, August 1988, pages 28-31. Thus, one of the boundary walls of each firing chamber consists of the semiconductor substrate (and typically one firing resistor). Another of the boundary walls of the firing chamber, disposed opposite the semiconductor substrate in one common implementation, is formed by a foraminous orifice plate. Generally, each of the orifices in this orifice plate is arranged in relation to a heater resistor in a manner in which enables ink to be directly expelled from the orifice. As the ink vapor nucleates inoculates at the heater resistor and expands, it displaces a volume of ink which forces a lesser volume of ink out of the orifice for deposition of the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of an ink feed channel in one of the boundary walls of the firing chamber.
As users of inkjet printers have begun to desire finer detail in the printed output from a printer, the technology has been pushed into a higher resolution of ink droplet placement on the medium. One of the common ways of measuring the resolution is the measurement of the maximum number of ink dots deposited in a selected dimension of the printed medium, commonly expressed as dots per-inch (dpi). The production of an increased number of dots per inch requires smaller droplets. Smaller ink droplets means lowered drop weight and lowered drop volume for each droplet. Production of low drop weight ink droplets requires smaller structures in the printhead. Merely making structures smaller, however, ignores the fact that complex interactions between the various structures make the optimization of a printhead design quite complex. Thus, it is desirable that an optimization be reached so that improved resolution may be realized with acceptable throughput and cost.
Conventionally, an orifice plate for a thermal inkjet printer printhead is formed from a sheet of metal perforated with a plurality of small holes leading from one side of the metal sheet to the other. There has also been increased use of a polymer sheet through which holes have been created by ablation or other means. In the metal orifice plate example, the process of manufacture has been well described in the literature. See, for example, Gary L. Siewell, et al., xe2x80x9cThe Think Jet Orifice Plate: A Part With Many Functionsxe2x80x9d, Hewlett-Packard Journal, May 1985, pages 33-37; Ronald A. Askeland, et al., xe2x80x9cThe Second-Generation Thermal Inkjet Structurexe2x80x9d, Hewlett-Packard Journal, August 1988, pages 28-31; and U.S. Pat. No. 5,167,776 xe2x80x9cThermal Inkjet Printhead Orifice Plate and Method of Manufacturexe2x80x9d.
It is axiomatic in thermal inkjet printer printheads that the orifice plate thickness be no less then approximately 45 microns thick. Orifice plates thinner then 45 microns suffer the serious disadvantage of being too flimsy to handle, likely to break apart in a production environment, or likely to become distorted by heat processing of the printhead. Orifice plates are typically manufactured by electroforming nickel on a mandrel and subsequently plated with a protecting metal layer.
A thick orifice plate (45 microns or thicker) generally requires a large heater resistor to provide the necessary force to expel a small droplet of ink past the relatively thick orifice layer and toward the medium. In comparison to small droplets: large structures such as these are inappropriate for those desired for high-resolution printing. U.S. patent application Ser. No. 08/920,478 xe2x80x9cReduced Size Printhead for an Inkjet Printerxe2x80x9d filed on behalf of Pidwerbecki, et al. on Aug. 29, 1997, offers one solution to the obtaining of an orifice plate. This solution, however, does not provide the total answer to the problem, particularly when the higher resolution demanded it of a printer requires further optimization of all of the structures of a printhead. It is desirable, therefore, that optimization of the structures of a printhead be optimized so that higher resolutions, resolutions equivalent to 600 dpi or greater, be developed and incorporated into a commercially practical printhead.
A printhead for an inkjet printer provides high-resolution printing by employing a substrate including at least one ink ejector on its surface and an orifice plate affixed to the substrate. The orifice plate has a plurality of orifices disposed through it from a first surface proximate the surface of the substrate to a second surface distal to the surface of the substrate. The orifice plate has a thickness in the range of 20 to less than 25 microns and at least two orifices of the plurality of orifices having centers at the second surface spaced apart by a distance having a range of 76 to 94 microns. Each of the at least two orifices has an orifice opening at the second surface with a diameter having a range of 12 to 14 microns.